java.lang.ArrayIndexOutOfBoundsException: 3

algorithm java coursera

when doing week4 assignment, Eclipse reports:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException:3
at Board.twin(Board.java:97)
at Board.main(Board.java:194)

What I found in oracle's javadoc:

public class ArrayIndexOutOfBoundsException
extends IndexOutOfBoundsException
Thrown to indicate that an array has been accessed with an illegal index. The index is either negative or greater than or equal to the size of the array.
Since:
JDK1.0

So, the problem is about the index of an array, check out your code like i=0;i<n;i++ , probably your index could be 4 while your array only have 4 elements, whose biggest index is 3. This exception indicates problems like this, be careful and patient.